Nowadays, the Third Generation Partnership Project (3GPP) is performing standardization work for a fourth generation mobile communication system. “Long Term Evolution (LTE)” that is one of data communication specifications developed by the 3GPP is a long-term advanced system aimed at the fourth generation (4G) IMT-Advanced and is also referred to as “3.9G (super 3G)”.
In LTE, it is possible to select two types of duplex scheme: Frequency Division Duplex (FDD) and Time Division Duplex (TDD). The FDD uses an uplink dedicated region and a downlink dedicated region. In each of an uplink and a downlink, a format of a radio frame which is configured with ten consecutive subframes is used. The uplink referred to herein is communication from a terminal station (UE terminal: User Equipment) to a base station (eNodeB: evolved NodeB) of LTE, and the downlink is communication from the eNodeB to the UE terminal. Also in the TDD, a format of a radio frame configured with ten consecutive subframes is used. However, the communication is performed using the same bandwidth in the uplink and the downlink in the TDD. Respective subframes constituting the radio frame are configured with Phy Downlink Control Channels (PDCCHs) used for control signals from the eNodeB and Phy Downlink Shared Channels (PDSCH) used for user data.
In specification developing work of Release 11 of LTE by the 3GPP, Coordinated Multi Point Transmission/Reception (CoMP) works as a key technology to improve a throughput of the UE terminal located in a cell edge. CoMP is a technology intended to increase power of a desired signal and to reduce interference from other cells, by a plurality of eNodeBs simultaneously transmitting and receiving data to and from a single UE terminal. In order to efficiently draw a micro diversity gain by CoMP, it is necessary to consider a pre-coding, a reference signal, and a measurement and feedback method. In Release 8 of LTE, a scheme of stratifying various sizes of cells such as Macro/Micro/Pico/Femto termed HetNet has been considered, but CoMP includes communication with such a femto cell. In addition, CoMP includes UpLink CoMP and DownLink CoMP and UpLink CoMP is an important technology similar to DownLink CoMP, but CoMP will be described as DownLink CoMP in the following description unless otherwise described.
A method of a cell for realizing CoMP includes a scheme in which a plurality of eNodeBs respectively perform autonomous distribution control, and a scheme in which one Macro eNodeB intensively controls a plurality of Pico eNodeBs. In the latter intensive control scheme, a plurality of base stations such as Remote Radio Heads (RRHs) as a measure for non-sensing in cell edges and the like are disposed, and the Macro eNodeB which intensively controls the base stations are connected with the base stations by baseband signals using an optical fiber (described later). Then, the Macro eNodeB collectively performs inter-cell radio resource control which performs a baseband signal process and control of respective RRHs.
With Respect to Determination of CoMP Set
A combination of eNodeBs performing CoMP for a certain one UE terminal, that is, constituting a cooperative group is referred to as a Set of CoMP transmission points, or, in short, a CoMP set, hereinafter.
In order to determine the CoMP set, it is necessary to determine which eNodeBs are effective for use, with respect to the UE terminal. This is referred to as a CoMP set determination or a point selection.
As a method of the point selection, there is a method in which the UE terminal receives reference signals from respective base stations and measures Reference Signal Received Power (RSRP) for the respective base stations so as to select eNodeBs with higher power as the CoMP set.
For example, a cooperative group setting method has been proposed in which a base station transmits a cooperative group setting signaling, to which Cell IDs of cells in a cooperative group selected for the user terminal are attached, to the user terminal, and the user terminal performs a cooperative group setting using the Cell IDs, which are attached to the cooperative group setting signaling, of the cells in the cooperative group selected for the terminal (for example, see PTL 1).
Further, a wireless communication method has been proposed in which the base station respectively assigns a portion of all frequency bandwidths to bandwidths respectively used in transmission by a single base station (first transmission scheme) and transmission by a plurality of base stations (second transmission scheme), and determines which transmission scheme is to be used based on feedback information indicating a reception quality which is received by either one, in other words, reduces a feedback information amount by limiting the feedback information to information indicating the reception quality of a portion of all frequency bandwidths (for example, see PTL 2).
Further, a wireless communication system has been proposed in which respective base station apparatuses receive quality information indicating communication qualities between a target terminal apparatus and base station apparatuses which can communicate with the target terminal apparatus from the target terminal apparatus and acquire schedule information indicating communication schedules between the base station apparatuses and terminal apparatuses present within cells of other base station apparatuses so as to select some base station apparatuses functioning as the base stations for the target terminal apparatus based on the quality information and the schedule information (for example, see PTL 3).
Request for Frequency of Point Selection
It is necessary to perform the point selection at each of regular time intervals (in other words, it is necessary to update the CoMP set at each of regular time intervals). In other words, there is a problem of updating semi-statically or dynamically the CoMP set of the UE terminals. In terms of the movement of the UE terminal, a dynamic point selection which dynamically updates the CoMP set is desirable.
Basic Matters to be Considered in Dynamic Point Selection
When the dynamic point selection is performed, it is necessary to consider a decrease in a throughput generated by the reference signal occupying a communication sequence within a system, in other words, an overhead of the downlink due to reference signals.
Further, it is necessary to consider an increase in power consumption of the UE terminal by the UE terminal frequently performing the measurement of the reference signals and the like.
Furthermore, it is necessary to consider an increase in the overhead of the uplink, due to the UE terminal feeding back the measurement results of the reference signals to the eNodeB.